In industry, the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced. The quality requirements are continuously raised and at the same time it is desired to reduce costs. The powder metallurgy (PM) technology with uniaxial pressing enables a cost effective production of components, especially when producing complex components in long series, as net shape or near net shape components can be manufactured without the need of costly machining. A drawback with the PM technology with uniaxial pressing is, however, that the sintered parts will exhibit a certain degree of porosity which may negatively influence the mechanical properties of the part. The development within the PM industry has therefore been directed to overcome the negative influence of the porosity basically along two different development directions.
One direction is to reduce the amount of pores by compacting the powder to higher green density (GD), facilitating sintering to a high sintered density (SD) and/or performing the sintering under such conditions that the green body will shrink to high SD. The negative influence of the porosity can also be eliminated by removing the pores at the surface region of the component, where the porosity is most harmful with regards to mechanical properties, through different kinds of surface densification operations.
Another development route is focused on the alloying elements added to the iron-based powder. Alloying elements may be added as admixed powders; fully pre-alloyed to the base iron powder; or bonded to the surface of the base iron powder through a so called diffusion bonding process. Carbon is normally admixed as graphite in order to avoid a detrimental increase of the hardness of the powder and decrease of compressibility if pre-alloyed. Other commonly used alloying elements are copper, nickel, molybdenum and chromium. The cost of alloying elements however, especially nickel, copper and molybdenum, makes additions of these elements less attractive. Copper will also be accumulated during recycling of scrap why such recycled material is not suitable to be used in many steel qualities where no, or a minimum of, copper is required. Chromium is more attractive due to low cost and excellent hardenability effect.
U.S. Pat. No. 4,266,974 discloses examples of alloyed powders outside the claimed scope containing only manganese and chromium as intentionally added alloying elements. The examples contains 2.92% of chromium in combination with 0.24% of manganese, 4.79% of chromium in combination with 0.21% by weight of manganese or 0.55% of chromium in combination with 0.89% by weight of manganese.
JP59173201 discloses a method for reduction annealing of a low alloyed steel powder containing chromium, manganese and molybdenum. One example shows a powder having a chromium content of 1.14% by weight and a manganese content of 1.44% by weight as the only intentionally added alloying elements.
A chromium, manganese and molybdenum based pre-alloyed steel powder is disclosed in U.S. Pat. No. 6,348,080.
WO03/106079 discloses a chromium, manganese and molybdenum alloyed steel powder having lower content of alloying elements compared with the steel powder described in U.S. Pat. No. 6,348,080. The powder is suitable to form bainitic structures at carbon content above about 0.4% by weight.
During recent years increased interest has been shown in the industry to produce components, such as gears and synchronization hubs for automotive applications, by the PM processes as such components are produced in long series and normally have a size and shape suitable for this manufacturing process. It has, however, been shown that there are difficulties to obtain sufficient strength and hardness for such components in order to withstand the harsh environment such components are subjected to. To overcome the problems, it has been necessary to apply additional process steps, such as surface densification, to obtain sufficient surface hardness and dimensional tolerances. Problems have also been encountered related to hardening of the sintered components, as the porosity in the components makes it difficult to control the case depth when conventional case hardening processes, by gas carburizing at normal pressure followed by quenching in oil, are applied. Furthermore, conventional case hardening of PM gears leads to problems with oxidation for powder materials that contain oxidation sensitive alloying elements, such as e.g. chromium. Thus, there is a need for improved materials and processes for the production of PM components aimed for stressful conditions.